Oscillation circuit



Augy- 15,- 1944- w. w. SHANNON I 2,355,606

' OSCILLATION CIRCUIT I Filed July 1; 1942 INPUT VOLTAGE CONTROL TU BE RESISTANCE FiaA- OSCILLATOR. m v VOLTAGE T Fie.5 m 0 PLATE CURRENT Fie.6 l L OUTPUT VOLTAGE.

Zhwentor Wil ham W. Shannon Gttornegs Patented Aug. 15, 1944 OSCILLATION CIRCUIT William W. Shannon, Troy, N. Y., assignor to W. & L. E. Gurley, Troy} N. Y., a corporation of New York Application July 1, 1942, Serial No. 449,277

3 Claims. (Cl. 25036) This invention relates to electron tube oscillation circuits and to methods of controlling the same. More particularly, it relates to a novel method of controlling the oscillation of a vacuum tube oscillator so as to make that oscillator operable as a triggering circuit or a pulse producer.

It has been suggested in the prior art to employ electron tube oscillators for the production of electrical pulses in measuring or control circuits, but for the most part the operation of such schemes has involved either the imposition of a mechanical or electrical load upon the circuit to be controlled, or else there has been an undesired variation in the accuracy of the circuit under extreme conditions. For example, in the measurement of frequency, extreme precision has been obtained in measuring frequencies of the order of 1500 cycles or more per second, but as the frequency decreased and approached zero, uncertainties and errors have crept in. In other words, while instruments of this character are capable of operating over a wide range of frequencies, there is a tendency as the frequency approaches zeroand the pulses are spaced further and further apart, for the instruments to tend to altertheir characteristics.

In the prior art when using electron tubes in triggering or pulse producing circuits, it has been suggested to control the oscillation of the tube by control of the feedback circuit of the oscillator, usually by interposing between the feedback coil and the grid coil a metal vane which by its shielding action caused a change in the condition of oscillation of the tube between two values, one in a range to cause oscillation and the other outside of the range for causing oscillation. The use of a metal vane in this manner, while satisfactory for many purposes, sometimes has the undesirable characteristic of imposing on the oscillation circuit some undesired reactions. In a pulse producing or triggering circuit it is highly desirable that the system have two characteristics, namely (1) an output function which is independent of the strength or intensity of the applied voltage which causes the oscillator to change from one condition to the other, but is dependent upon the frequency of that voltage, and (2) the change of state between non-oscillation and oscillation must be rapid and constant and be independent of the number of such changes per unit of time. Prior art devices have been deficient in one or the other of these characteristics.

The present invention has both of the characteristics referred to and also has the desirable faculty of causing the pulses to cut off sharply even at extremely low frequencies of input voltage. 7

Accordingly, the applications of the present invention are of wide'scope and embrace numerous fields which need not be mentioned specifically.

In general it may be said that it will find its greatest application in'the control of oscillating circuits such as are used for their triggering or pulse producing action, as in integrators, frequency and speed measuring devices, control circuits'depending upon triggering action, control or measuring circuits in general designed to be operated in response to electrical pulses of predeter mined frequency or wave form, and to any oscillation circuit where it is important to electrically isolate the oscillation circuit from the electrical source which causes the changes of state in that circuit. 5

In describing the invention,"a preferred embodiment will be illustrated as applied to the con{ trol of a conventional oscillation circuit, giving only an abstract reference to the sensing or con-' trol means to which the pulses supplied by the circuit are delivered.

In the drawing: V

Figure 1 is a circuit diagram showing an embodiment of the invention wherein an oscillation circuit is controlled by varying the plate resist ance of an electron tube upon the control element' of which there is applied a varying potential;

Figs. 2 to 6, inclusive, are curves showing the values of electrical quantities prevailing at a given instant at indicated points in the circuit of Fig. 1. For example, Fig. 2 shows the wave form of one type of input voltage which may be applied to the circuit at point A. The curves are drawn to a common horizontal time scale to correlate the various quantities with respect to time.

In Fig. 1, reference character I designates an oscillator tube inthe form of a triode having a parallel tuned circuit formed of a coil 8, and a condenser 9 connected in its grid circuit. The plate circuit of the oscillator includes a feedback coil II in inductive relation to the grid coil 8 for setting up oscillations of a, frequency determined by the constants of the tuned circuit. Plate current for the tube 1 is supplied from a suitable source as indicated by the positive terminal at l2 through a current limiting resistance l3 and an inductance It. A condenser I5 is connected between ground and a point between inductance M and feedback coil II. The theory of operation of this condenser is not clearlyunderstood, but when its value is kept low, improved operation of the system results. The oscillation circuit of' the tube includes the usual grid biasing condenser IS with a ground connection at I! through resistor l8.

As is well understood in the art, this circuit is capable of serving as a source of electrical oscillations by transfer of energy from the feedback coil H to the grid coil 8, so that alternating current of a frequency determined by the fixed characteristics of the tuned circuit is supplied to the output circuit of the tube.

No novelty is claimed in this application for the details of the oscillation circuit, the invention being directed solely to an arrangement for controlling such a circuit, making it useful as a sourc of electrical pulses capable of wide and varying application.

As is well known, it is possible by the interpositioning of a magnetic field disturbing element between the coils 8 and II to control the oscillation of the triode. For example, it is possible to interpose a metal vane between these coils, and with the vane interposed the oscillation of the tube is stopped. Similarly, when the vane is removed from the magnetic field interlinking the coils 8 and II, the oscillations start. The use of a metal plate has certain inherent characteristics which are undesirable in the production of electrical pulses, particularly those of extremely low frequencies approaching zero. As the frequency of these pulses: drops to a few cycles per second, the cut-off of the pulses in the output circuit ceases to be sharp and precise, and consequently any measurement based on the periodicity of those pulses, or on an integration of the total energy supplied by the pulses tends to decrease in accuracy as the frequency closely approaches zero.

The present invention is based on the discovery that precise pulsing may be produced in the output circuit of an oscillator tube by introducing into the feedback connection an electrical energy absorbing or reacting circuit which is capable of control between values above and below that necessary in producing the conditions for oscillation of the oscillator tube.

While the precise theory underlying the operation of the present control is not at present fully understood, it is believed that the operation of this control can be explained on the basis of either energy absorption in the controlling circuit or phase relation of the currents flowing in that circuit and theassociated feedback circuit of the oscillator. It is essential that some means be provided for varying the electrical constants of a control circuit which includes a coil disposed in energy absorbing relation with the feedback circuit of the oscillator. Preferably, the coil is in inductive relation to that feedback circuit as shown in Fig. 1. In that figure, reference character I!) designates such a coil disposed in inductive relation to the coils 8 and H. This coil is shunted by a condenser 2| and one terminal leads through condenser 22 to the plate circuit of the three element electron tube 23. Positive potential is applied to the plate circuit of the tube 23 through a current limiting resistor 24. The input connections to the control tube 23, designated 25 and 26, are shunted by a resistor 21. The common portion of the filament and plate circuits of the tube 23 includes a resistance 28.

The constants of the circuit of tube 23 are preferably so chosen that when the grid of tube 23 is slightly negative the resistance of the plate circuit of the tube rises to such a value as to permit oscillation of the triode 1. When, on the other hand, the characteristics of the input energy are such as to impress a slightly positive potential on the grid of tube 23, the plate resistanc of that tube drops to a value to stop oscillation of the triode. Through a suitable choice of circuit constants it becomes possible in this way to make the starting and stopping of oscillation of triode 1 critical, and to change it from a state of oscillation to one of non-oscillation merely by a change in polarity of the grid of tube 23, or by a change in the value of voltage applied to the grid of tube 23.

For best operation it is preferred that the coil 89 be so tuned by means of condenser 2| as to approximate the output frequency of the triode l. The coil 19 and the condenser 2! thus constitute a parallel resonant circuit. If the resonant frequency of that tuned circuit is below that of the oscillator frequency, the tuned circuit becomes practically a pure capacity reactance. If, on the other hand, the resonant frequency of the tuned circuit of coil I9 is above the oscillator frequency, the tuned circuit becomes practically a pure inductive reactance. Variation of the impedance of tube 23 afiects the tuning of the circuit of coil l9, and by proper choice of constants of this circuit, changes in impedance of tube 23 will cause a shift in phase of the voltage fed back to coil 8 and the shift in phase may be made to occur in a direction to aid in stopping or starting oscillations. The ex act theory of operation of the oscillation circuit is not fully understood; the control of the oscillation condition may be due to a variation in the amount of energy absorbed by the circuit of coil I9, or it may be due to a variation in the phase relation of the feedback voltage supplied to the coil 8, or to-a combination of both of these effects. Any suitable responsive instrument such as that indicated diagrammatically at 31, grounded at 29, may be utilized for responding to the pulses delivered from the output circuit of triode 1 during its operation.

The conditions existing in the oscillation circuit and its control are exemplified by the curves of Figs. 2 to 6. Each curve, as indicated by the legend, shows the approximate conditions which may apply in corresponding parts of the circuit when the circuit is operating. If, for example, it be assumed that an input voltage having a wave form similar to that shown in Fig. 2 is impressed upon the input terminals 25 and 26, the curve showing the corresponding variation in the plate resistance of control tube 23 may have the form shown in Fig. 3. As indicated in Fig. 3, when the resistance of the control tube plate circuit is above a certain value, as indi cated by the dark area of the curve above the dotted horizontal line, oscillation of the triode will stop, whereas during other portions of the cycle it will continue.

The voltage delivered from the output circuit of the oscillator and supplied to the instrument or device 3| is indicated in Fig. 4, it being noted that the amplitude of the output voltage remains constant during oscillation of the tube and changes abruptly from maximum value to minimum value at the end of the oscillating period, giving rise to a plate current of flat top form as shown in Fig. 5. Fig. 6 indicates the resulting voltage applied to the device 31 in the form of sharp pulses which die away rapidly from maximum to zero value.

No precise application of the invention has been shown because the application is general and may find use in any oscillation circuit where the output pulses are to be employed for purposes of measurement, summation, control, indication, or the like. While the control tube 23 has been shown as a three element triode of the vacuum type, it is obvious that any electron tube having the characteristics recited may find ap plication in this circuit and regardless of the number of elements employed or the presence or absence of gas.

What is claimed is:

1. An oscillation control circuit comprising a vacuum tube oscillator having input and output circuits; a feedback connection between said input and output circuits; a control tube of the vacuum type having at least three elements including a filament, a grid and a plate; a circuit tuned to the output frequency of the oscillator, said circuit including an inductance coil in the plate circuit of the control tube and disposed in inductive relation to said feedback connection; and means for impressing a varying potential upon the grid of the control tube to vary the resonant frequency of said inductance coil circuit abruptly between values above and below oscillator frequency to vary the condition of oscillation of said oscillator between oscillation and non-oscillation values. 7

2. An oscillation control circuit comprising a vacuum tube oscillator having input and output circuits; a feedback connection between said input and output circuits; a control tube of the vacuum type having at least three elements including a filament, a grid and a plate; a circuit tuned to the output frequency of the oscillator, said circuit including a parallel resonant circuit having an inductance coil in the plate circuit of the control tube and disposed in inductive relation to said feedback connection; and means for impressing a varying potential upon the grid of the control tube to vary the resonant frequency of said inductance coil circuit between values above and below oscillator frequency to vary the condition of oscillation of said oscillator between oscillation and non-oscillation values.

3. An oscillation control circuit comprising a vacuum tube oscillator having input and output circuits; a feedback connection between said input and output circuits; a control tube of the vacuum type having at least three elements including a filament, a grid and a plate; a parallel resonant circuit tuned to the output frequency of the oscillator, said circuit having an inductance coil inductively related to said feedback connection; and means for varying the control characteristics of the grid of the control tube to vary the resonant frequency of said inductance coil circuit abruptly between values above and below oscillator frequency to vary the condition of oscillation of said oscillator between oscillation and non-oscillation values.

WILLIAM W. SHANNON. 

